1. Field of the Invention
The present invention relates to carrier tracking loops (CTLs) and, in particular, to CTLs for use in direct sequence spread spectrum (DSSS) systems.
2. Description of the Related Art
Digital data transmission from a transmitter to a receiver requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered or acquired by the receiver. In digital wireless telephone systems, for example, a wireless (cordless) telephone handset unit communicates via digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. Each handset and the base comprise a transceiver, having a transmitter and receiver. In such a system, a user may employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are in use in various situations, such as businesses with many telephone users. Such systems employ a base unit that communicates with up to N handsets in real time, typically with digital communications schemes, such as a spread-spectrum, time division multiplex (TDM) schemes such as time division multiple access (TDMA). In a spread spectrum system, bandwidth resources are traded for performance gains, in accordance with the so-called Shannon theory. The advantages of a spread-spectrum system include low power spectral density, improved narrowband interference rejection, built-in selective addressing capability (with code selection), and inherent channel multiple access capability. Spread-spectrum systems employ a variety of techniques, including direct sequencing or sequence (DS), frequency hopping (FH), chirp systems, and hybrid DS/FH systems. DS spread spectrum systems are sometimes referred to as DSSS systems.
In a TDMA system, a single RF channel is used, and each handset transmits and receives audio data packets as well as non-audio data packets during dedicated time slices or time slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiple access (CDMA), and combinations of such schemes. Various modulation schemes are employed, such as carrierless amplitude/phase (CAP) and quadrature amplitude modulation (QAM).
Digital data is typically transmitted as modulated signals over a transmission medium, such as the RF channel. (Other transmission media often used for digital communications include asymmetric digital subscriber loop (ADSL) systems or cable modem systems.) The digital data, in the form of a stream of binary digits (bits), is first mapped to a stream of symbols, each of which may represent multiple bits. A constellation is the set of all possible symbols for a given signaling scheme. Symbols can be a set of real amplitude levels, as in pulse amplitude modulation (PAM), or a set of points on a circle in the complex plane such as in quadrature phase shift keying (QPSK: 4 points on a circle, separated by 90 degrees of phase), or an array of points at different amplitudes and phases on the complex plane, as in QAM. Sets of bits are mapped to symbols by a look-up table (e.g., a ROM). The number of symbols in a signaling constellation depends on the encoding scheme. For example, each QPSK symbol represents 2 bits of the input data stream, with the 4 symbols, 1+j, 1xe2x88x92j, xe2x88x921+j, xe2x88x921xe2x88x92j each representing the bit patterns 00, 01, 10, and 11, respectively. The real portion of such complex digital symbols is referred to as in-phase, or xe2x80x9cIxe2x80x9d data, and the imaginary part as quadrature, or xe2x80x9cQxe2x80x9d data, yielding I, Q pairs.
To transmit a given input data value in a complex data system, the input data value to be transmitted is mapped to a symbol pair or pair of coordinates I,Q of a corresponding constellation point on a complex signal constellation having real and imaginary axes I and Q. These I,Q symbols, which represent the original data value, are then transmitted as part of data packets by a modulated channel. A receiver can recover the I, Q pairs and determine the constellation location therefrom, and perform a reverse-mapping to provide the original input data value or a close approximation thereof.
In a DSSS type spread spectrum system, each symbol is transmitted by a string of xe2x80x9csub-symbolsxe2x80x9d or xe2x80x9cchipsxe2x80x9d, which is typically derived by multiplying the symbol (which may be either a 1 or xe2x88x921, in some schemes) times a pseudo-random number (PN) binary string of a certain length (number of chips C). Such systems are thus characterized by a chip rate, which is related to the symbol rate. Spread spectrum systems may also be used, in general, to transmit any digital data, whether in complex format or not, and whether or not in a TDMA system.
Thus, in a DSSS system, a signal represents successive symbols, by means of successive xe2x80x9cchipsxe2x80x9d of symbols. A received signal is sampled to provide samples. Samples thus represent a signal, which itself represents chips, which represent symbols.
The receiver side of a transceiver samples a received signal with an analog-to-digital converter (ADC), which provides samples representative of the signal, which in turn represents symbols. The transmitter side of a transceiver converts symbols into analog samples that constitute a signal, with a digital-to-analog converter (DAC).
As noted above, digital data transmission requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter (e.g., the transmitter of the base unit transceiver) and successfully recovered by the receiver (e.g., the receiver of a given handset transceiver). For example, the receiver side of a data transmission in a spread-spectrum digital wireless telephone systems employs a variety of functions to recover data from a transmitted RF signal. These functions can include: timing recovery for symbol synchronization, carrier recovery (frequency demodulation), and gain. The receiver thus includes, inter alia, an automatic gain control (AGC) loop, carrier tracking loop (CTL), and timing loop for each link.
Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multituple of the transmitter symbol (or chip) rate. For example, some receivers sample the received signal at twice the transmitter symbol (or chip) rate. In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter. Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. AGC tracks signal strength and adjusts the gain, for example to help compensate for the effects of transmission channel disturbances upon the received signal. AGC, along with other equalization techniques, can help remove intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. These and related functions, and related modulation schemes and systems, are discussed in greater detail in Edward A. Lee and David G. Messerschmitt, Digital Communication, 2d ed. (Boston: Kluwer Academic Publishers, 1994).
In a burst mode or TDMA communication system, such as a TDMA-based multi-line wireless telephone system, quick acquisition of carrier loops is required to efficiently utilize available bandwidth. For example, a TDMA-based digital multi-line wireless telephone system may use a TDMA audio packet structure such as structure 200 illustrated in FIG. 2, where a base unit having a transceiver sequentially transmits to and receives from different handsets over the time interval Td, with guard time Tg between packet transmissions. Guard time is established to allow the transmitters to power-down and to allow the receivers to power-up. The receivers must synchronize for each packet. During synchronization, data is unreliable, so the system bandwidth efficiency is reduced because of time being used to synchronize the system. It is, therefore, inportant to minimize or reduce this synchronization time, i.e. to provide for quicker acquisition of carrier loops.
Thus, in a DSSS multiline wireless telephone system, as in all spread spectrum systems, it is important for each transceiver in the system to be able to accurately and quickly receive transmitted signals and, in particular, to provide for accurate and quick carrier tracking. There is, therefore, a need for improved techniques for carrier tracking in spread spectrum communications systems and, therefore, for improved CTLs and CTL techniques.
A wireless telephone system comprises a base transceiver having a base receiver and a plurality of wireless handsets. Each handset comprises a handset transceiver for establishing a DSSS link over a shared channel with the base unit via the base transceiver. Each receiver the base transceiver and the handset transceivers receives a spread-spectrum signal representing symbol data from a transmitter of the system, where each such receiver comprises a derotator that derotates the spread-spectrum signal in accordance with a counter-rotating signal to provide a derotated signal; a correlator for receiving the derotated signal and for providing output symbol data based on the derotated signal; a carrier tracking loop (CTL) phase error estimator for receiving the output symbol data and for generating a CTL phase error signal based upon the rotation of the spread-spectrum signal; and a CTL for generating the counter-rotating signal based on the CTL phase error signal.